1. Field of the Invention
The present invention relates to the field of displaying technology, and in particular to a pixel structure and a manufacturing method thereof.
2. The Related Arts
The displaying technology has undergone rapid progress recently. A flat display device applies totally different techniques of displaying and manufacturing, making it significantly differing from the conventional video image displaying devices. The conventional video image displaying device is generally based on a cathode ray tube (CRT), from which a flat display device is made different primarily concerning changes made in respect of weight and size (thickness). Generally, a flat display device has a thickness not greater than 10 centimeters, among the other differences associated with various technical aspects, such as theory of displaying, manufacturing material, manufacturing process, driving for displaying video images.
A liquid crystal display (LCD) is one of the flat display devices that are most commonly used and comprise a color screen of high PPI (pixels per inch) and is widely used in various electronic equipment, such as a mobile phone, a personal digital assistant (PDA), a digital camera, a computer monitor, and a notebook computer screen.
A currently available liquid crystal display is generally composed of upper and lower substrates and a central liquid crystal layer and the substrates are each composed of a piece of glass and electrodes. If the upper and lower substrates are both provided with electrodes, then a liquid crystal display of a longitudinal electrical field mode can be provided, such as a TN (Twist Nematic) LCD, a VA (Vertical Alignment) LCD, and MVA (Multi-domain Vertical Alignment) LCD that is developed for addressing the issue of narrow viewing angle. Another category is different from the above described LCDs and comprises electrodes that are provided only on one side of the substrates so as to form an LCD of a lateral electrical field mode, such as an IPS (In-Plane Switching) LCD and an FFS (Fringe Field Switching) LCD. The FFS LCDs are widely used in various mobile communication devices due to high aperture ratio, high PPI, and wide viewing angle.
Currently, the display screens of the mobile communication devices are developed toward high PPI, high gamut value, high contrast, and low power consumption. With the increase of PPI, parasitic capacitance inside the screen becomes severer. To reduce the parasitic capacitance of the screen, a common practice is to increase the thickness of an insulation layer that is present between electrodes and made of silicon nitrides (SiNx) or silicon dioxide (SiO2) or to adopt an organic insulation layer that has an even greater thickness. Such practices may also lead to the reduction of beneficial capacitances, such as storage capacitor Cst, while reducing the harmful parasitic capacitance.
Specifically, reference is now made to FIGS. 1 and 2, wherein FIG. 1 shows the structure of a conventional pixel used in a mobile phone screen and FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1. To simplify the illustration, the structure a TFT (Thin-Film Transistor) related part is omitted in both FIGS. 1 and 2. In such a pixel structure for an FFS LCD, a parasitic capacitance designated with circled 1 between a transparent conductive layer electrode 100 of a common electrode and a data line 200 will increase RC delay of the data line 200 (namely signal transmission speed in the data line being affected by the multiplication of resistance (R) and capacitance (C)). This would cause insufficient charging of some pixels of a liquid crystal panel thereby displaying incorrect gray level and affecting displaying quality. To reduce the parasitic capacitance designated by the circled 1, it is common to increase the thickness of an insulation layer between the transparent conductive layer electrode 100 of the common electrode and the data line 200. However, this also reduces the storage capacitance, designated by circled 2, between the transparent conductive layer electrode 100 of the common electrode and a transparent conductive layer electrode 30 of a pixel electrode. According to the formula, ΔV=[Cgs/(Cgs+Cst+Clc)](Vgh−Vgt) (where Clc is the capacitance generated by a liquid crystal box, Cst is the storage capacitance, Cgs is the value of coupling capacitance between a gate terminal and a drain terminal of a thin-film transistor, Vgh−Vgt is voltage change on the gate terminal), the reduction of the storage capacitance, designated by circled 2, will increase the feed-through voltage, lowering down the brightness of the liquid crystal panel and reducing penetration.
Referring to FIG. 3, which illustrates a flow chart of the manufacture of a conventional pixel structure of an FFS liquid crystal display, the first method is: sequentially depositing a first metal layer (GE), a gate insulation (GI) layer, an amorphous silicon (a-Si) layer, a pixel electrode (Pixel ITO), a second metal layer (S/D), a passivation (PV) layer, and a common electrode (Com. ITO) on a glass substrate. The second method is: sequentially depositing a first metal layer (GE), a gate insulation (GI) layer, an amorphous silicon (a-Si) layer, a second metal layer (S/D), a pixel electrode (Pixel ITO), a passivation (PV) layer, and a common electrode (Com. ITO) on a glass substrate.